| dc.description.abstract | InSb single crystals were grown by the vertical and horizontal Bridgman techniques. About 81 experimental runs were carried out altogether (for 8 mm, 10 mm and 20 mm diameter InSb crystals). Among them, as many as 47 runs were unsuccessful due to various reasons such as void formation, multigrain formation (due to an air gap at the tip of the ampoule), ampoule cracking, low mobility values, etc. Many of these problems were overcome by employing various conditions such as preheating the ampoule during sealing, slower cooling rate, improved synthesized material, etc.
It was observed that the quality of crystals grown by both vertical and horizontal Bridgman techniques is comparable, but large size crystals with circular cross section were not possible with the horizontal Bridgman setup built in our laboratory. Hence, large InSb crystals with circular cross section were grown only using the vertical Bridgman technique.
During the course of studying these materials, we acquired considerable understanding of their structural, optical and transport properties, which are very sensitive to impurities present in the starting materials. We realized that unintentional impurities present in the material are the main cause of high carrier concentration and low electron mobility. Therefore, it is extremely important to start with high purity materials, and this has been achieved with 20 mm diameter crystals, which exhibited room temperature mobility, carrier concentration and energy gap values of 67,280 cm²/V·s, 2.1 × 10¹ cm ³ and 0.17 eV, respectively, with high structural perfection. Hence, these crystals can be used for IR detector applications. These crystals have been used for developing a Hall probe (in the laboratory, described in Chapter 2) and for further studies on engineering their various physical parameters.
We implanted InSb wafers with 50 MeV Li³ ions at various fluence levels from 2.6 × 10¹² to 1.6 × 10¹ ions/cm² at room temperature. The maximum change in optical and electrical properties is observed in samples irradiated with a dose of 1.6 × 10¹ ions/cm². As a result, the carrier concentration decreases from 8.5 × 10¹ to 1.1 × 10¹ cm ³, and the mobility increases from 5.4 × 10 to 1.67 × 10 cm²/V·s for 1.6 × 10¹ ions/cm² fluence. It is also observed that even at this fluence, there is not much change in crystalline quality.
It is concluded that defects produced due to irradiation cause a reduction in carrier concentration and the formation of electrically inactive complexes, which leads to an increase in mobility. Hence, Li ion irradiation under optimum conditions could be useful for device applications.
In conclusion, we have grown single phase InAs Sb (up to 5 at.% As) and InBi Sb (up to 6 at.% Bi) crystals using the Rotatory Bridgman method. Both these crystals are radially homogeneous, but in InBi Sb crystals, the Bi composition ratio increases along the growth direction.
XRD and TEM studies reveal a compressed lattice for InAs Sb and an expanded lattice for InBi Sb compared to InSb. FTIR transmission spectra at room temperature show a shift in the energy gap in both these crystals. This shift is larger in InBi Sb compared to InAs Sb , but the quality of InAs Sb crystals is better in terms of structural and electrical properties. Both crystals show n type behaviour, indicating donor nature of As and Bi in InSb.
Room temperature cut off wavelengths of 8.3 µm and 10.5 µm, along with high mobility and low background doping, make these crystals useful for long wavelength infrared detectors.
We encountered several problems during growth, especially at higher As and Bi concentrations. Most of the time, two phases or multigrain formation was observed. These problems were overcome using ACRT (like rotation) and thermally assisted diffusion. Due to phase separation and non uniform composition, growth with more than 5 % As and 6 % Bi was not possible.
In InBi Sb crystals, striations remained a major problem when grown from In, Bi, Sb source materials and under G/v values (~210 °C·hr/cm²) used for InSb growth. Through extensive experimentation, we found that lower G/v values (~40 °C·hr/cm²) yield higher quality crystals. Changing source materials to InSb and InBi also reduced mechanical stress issues.
For InAs Sb crystal growth, diffusion dominated conditions were attempted.
To conclude, we have shown that InSb films with minimum thickness of 3 µm can be epitaxially grown on 14.6% lattice mismatched (001) GaAs substrates by liquid phase epitaxy (LPE). Detailed structural (XRD, SEM, TEM), electrical (Hall) and optical (room temperature and low temperature IR transmission) studies show that the films are of high quality. In particular:
the films are single crystalline,
the interface is abrupt and uniform,
electron mobility is high,
transmission spectra match bulk InSb.
Hence, LPE can be an inexpensive and viable alternative for industrial growth of such heterostructures for IR detector applications.
We have also grown InAs Sb /GaAs and InBi Sb /GaAs heterostructures up to 6 at.% As and 4 at.% Bi by LPE. Growth parameters were optimized to obtain layers with good interface morphology (SEM). HRXRD on the epilayers showed distinct layer and substrate peaks for several reflections, indicating structural coherence. A 360° phi scan in asymmetric reflections gave four sharp peaks with negligible scattering, confirming single crystal nature. The grown layers are ~97% relaxed.
Optical studies reveal that room temperature cut off wavelengths lie within the 8–14 µm atmospheric window. The films are n type with high electron mobility (~10 cm²/V·s) and low carrier concentration (~10¹ cm ³). The quality is comparable to layers grown by MOCVD and MBE.
Although the band gaps of InAs Sb /GaAs and InBi Sb /GaAs are reduced compared to InSb (0.17 eV), the reduction is smaller than theoretical predictions. A possible reason is that not all As and Bi substitute Sb sites; some may occupy interstitial positions.
Hence, growth of InBi . As . Sb . /GaAs heterostructures was also carried out using LPE. These films are oriented and single crystalline with abrupt interface. Composition ratios (from Ar etching) show stable In/Sb ratios; As/Sb and Bi/Sb ratios stabilize after ~5 minutes of etching. Optical study reveals room temperature cut off wavelengths of 10.33–10.97 µm, higher than those of InBi Sb and InAs Sb films.
Regarding thin film growth (Chapters 6 and 7), the following general observations can be made:
InSb and InSb based ternary films were grown on GaAs by the ramp cooled LPE technique. Since the driving force ( µ) T/T , we regulated growth by changing T and T (T = initial growth temperature; T = undercooling). Initially, starting solutions were made using elemental In and Sb, but this was later replaced by InSb due to weighing inaccuracies with small Sb quantities.
The choice of solution composition and growth temperature is critical. In was chosen as the solvent due to negligible volatilization. High temperatures increase Sb volatility, while low temperatures increase melt viscosity and reduce substrate wetting. We experimentally determined optimal temperatures, avoiding In inclusions caused by constitutional super cooling at low temperatures and substrate degradation (As depletion) above 453 °C.
Growth was therefore carried out between 325–453 °C, with best films grown at optimized melt saturation temperatures:
423 °C (InSb), 351 °C (InBi Sb ), 446 °C (InAs Sb ).
Film quality was strongly dependent on growth rate. X ray diffraction shows that high cooling rates and large T lead to multigrain formation. Crystallinity improves markedly at lower growth rates. Films grown at 0.2 °C/hr showed only the (100) InSb peak, indicating highly oriented single crystal growth. A normalized intensity ratio for the (400) reflection (Table 6.1) quantifies this trend, showing that large growth rates promote off (100) nucleation. | |
